Two-layer golf balls are typically made with a single solid core encased by a cover. These balls are generally most popular among recreational golfers, because they are durable and provide maximum distance. Typically, the solid core is made of polybutadiene cross-linked with zinc diacrylate and/or similar crosslinking agents. The cover material is a tough, cut-proof blend of one or more materials known as ionomers, such as SURLYN®, sold commercially by DuPont or IOTEK®, sold commercially by Exxon.
Multi-layer golf balls may have multiple core layers, multiple intermediate layers, and/or multiple cover layers. They tend to overcome some of the undesirable features of conventional two-layer balls, such as hard feel and less control, while maintaining the positive attributes, such as increased initial velocity and distance. Further, it is desirable that multi-layer balls have a “click and feel” similar to wound balls.
Additionally, the spin rates of golf balls affect the overall control of the balls in accordance to the skill level of the players. Low spin rates provide improved distance, but make golf balls difficult to stop on shorter shots, such as approach shots to greens. High spin rates allow more skilled players to maximize control of the golf ball, but adversely affect driving distance. To strike a balance between the spin rates and the playing characteristics of golf balls, additional layers, such as intermediate layers, outer core layers and inner cover layers are added to the solid core golf balls to improve the playing characteristics of the ball.
The patent literature discloses a number of multi-layer golf balls. U.S. patent application Ser. No. 10/773,906 which is commonly owned and incorporated herein by reference in its entirety, is directed to an improved multi-layer golf ball displaying certain spin profile. The ball has a generally rigid, thermosetting polybutadiene outer core surrounding a relatively soft, low compression inner core. The inner core has a hardness that is less than the hardness of the outer core, and a specific gravity that is less than or equal to the specific gravity of the outer core. The inner core and outer core are formulated to provide a combined overall core compression of greater than about 50.
U.S. patent application Ser. No. 09/853,252, which is commonly owned and incorporated by reference in its entirety, is directed to golf balls having a cover comprising three or more layers: an inner cover layer, an outer cover layer, and an intermediate cover layer. The outer cover layer comprises a composition formed of a reactive liquid material, and the combination of the thickness of the cover layers is about 0.125 inch. Golf balls prepared accordingly can exhibit substantially the same or higher coefficient of restitution (“COR”), with a decrease in compression or flexural modulus, compared to golf balls of conventional construction. The resultant golf balls typically have a COR of greater than about 0.7 and an Atti compression of at least about 40.
U.S. patent application Ser. No. 10/279,506, which is also commonly owned, and incorporated by reference in its entirety, is directed to a golf ball comprising an inner core, an outer core, and a cover. At least a layer of the golf ball is made from a low compression, high COR material, and is being supported by a low deformation, high compression layer. The resulting golf ball has high COR at high and low impact speeds and low compression for controlled greenside play.
U.S. Pat. No. 6,645,089 to Tsuoda et al. and U.S. Pub. patent application Nos. 2002/0019268 and 2002/0042308 by Tsunoda, et al. are directed to a golf ball comprising a 6-layer core. The modulus of elasticity of each layer of the core progresses from lower to higher modulus in the direction from the center to the outermost core layer.
U.S. Pat. No. 6,419,595 to Maruko et al. is directed to a 5-piece golf ball comprising a single core and 4 cover layers. The innermost cover layer has less than 60 Shore D hardness, the next cover layer has greater than 45 Shore D hardness, and the outermost cover layer is harder than the third cover layer.
However, there remains a need to provide multi-layer golf balls having velocity gradient that progresses from a slower center to a faster cover to match the balls to the players' swing speed.